1. Field of the Invention
The present invention is directed to a method and to a magnetic resonance tomography apparatus for spatially resolved presentation of a change in the functional activities of a brain of a living subject under observation by means of magnetic resonance.
2. Description of the Prior Art
It is known that brain activities in the cerebral cortex of human beings elicited by external stimulation can be detected with magnetic resonance tomography. Such an external stimulation can be a visual or an acoustic stimulus, for example.
An experiment typically implemented with magnetic resonance tomography is referred to as the fMRI-BOLD experiment. xe2x80x9cfMRIxe2x80x9d stands for functional magnetic resonance imaging and BOLD stands for xe2x80x9cblood oxygenation level dependentxe2x80x9d. A goal of functional magnetic resonance tomography (imaging) is to exactly detect the activity changes in the brain of the living subject under observation that are attributed to a specific stimulation. Before the surgical removal of a brain tumor, for example, it is thus possible to demarcate functional centers of the brain from the tumor in order to avoid damage to functionally important regions of the brain of the patient.
The BOLD effect is based on the different magnetic properties of oxygenated and de-oxygenated hemoglobin in the blood. In contrast to the diamagnetic oxyhemoglobin, desoxyhemoglobin has two unpaired iron electrons and is therefore paramagnetic. An increase in the local concentration of desoxyhemoglobin as a result of a local brain activity (neural activity) therefore leads to an non-homogeneous local magnetic field. This accelerates the decay of the imaging cross-magnetization of nuclear spins excited by means of a magnetic resonance tomography apparatus. Given intensified brain activity, the increased oxygen need associated therewith is over-compensated by an increased delivery of diamagnetic oxyhemoglobin. Gradient echo sequences of the magnetic resonance tomography apparatus, which react especially sensitively to local field inhomogeneities, therefore exhibit a weak intensity boost in the magnetic resonance image given intensified brain activity.
A crucial problem in functional magnetic resonance tomography is thus to separate brain activities elicited by a specific stimulation from other brain activities.
For solving this problem, it is known to calculate a correlation coefficient between a stimulation function employed for stimulation and the obtained, temporal signal curve of a pixel (picture element) for each pixel of time-successive magnetic resonance images (frequently several hundred) of the brain of the living subject under observation. Mathematically formulated, a determination is made for each pixel as to whether there is a significant relationship between the time curve of the stimulation function and a brightness fluctuation of the pixel.
This means that the time curve of the stimulation function, referenced to the magnetic resonance images of the brain of the living subject under observation that are produced, must be known before the implementation of the correlating. Periodic functions therefore are usually employed as stimulation functions. A typical stimulation function is a periodic sequence of stimulations separated by pauses (for example, 20 sec. of finger movement, 30 sec. of rest, 20 sec. of finger movement, 30 sec. of rest, . . .).
A disadvantage of the above-described correlation is that an exact knowledge of the stimulation function is required so that cognitive processes being examined can be detected.
An object of the present invention is to provide a method and a magnetic resonance tomography apparatus that enable a detection of changes in the functional activities of a brain of a living subject under observation in a simple way and with high precision, even without knowledge of a time curve of the (usually external) stimulation.
This object is achieved according to the present invention, in a method for spatially resolved presentation of a change of functional activities in the brain of a living subject under observation by means of magnetic resonance tomography wherein temporally successive magnetic resonance images of the brain of the living subject stimulated by a stimulus (external or otherwise) are obtained upon variation of at least one of the excitation angle and the echo time from image to image, a noise part is calculated for each pixel referenced to identical pixels of the temporally successive images, the noise part of each pixel is resolved into a first noise component independent of the excitation angle and a second noise component dependent on the excitation angle, the second noise component of the noise part of each pixel is resolved into a third noise component independent of the echo time and a fourth noise component dependent on the echo time, and the fourth noise component of the noise part of each pixel is employed for detecting neural activity changes in the brain of the living subject under observation.
Even without knowledge of the time curve of the stimulation of the living subject and without implementing a cross-correlation, a t-test or some other statistical method operating by means of reference function, the inventive method makes it possible to detect neural activity changes in the brain of a living subject under observation.
Consequently, cognitive processes or activity changes in the brain of the living subject that can be attributed to statistical processes can be detected with the inventive method.
Since this detection inventively ensues for each pixel, the detected activity changes can be presented spatially resolved. Since the noise components that are independent of the excitation angle and the echo time do not contribute to the detection of the pixels representing activity changes that are sought, moreover, an especially high sensitivity of the activity changes in the brain of subject can be achieved.
The step of calculating a noise part referenced to identical pixels of the temporally successive images for each pixel preferably ensue by calculating the standard deviation of the signal curve for each pixel in the individual images that are produced.
The step of resolving the noise part of each pixel into the first noise component independent of the excitation angle and the second noise component dependent on the excitation angle preferably ensues by observing at least two images produced using different excursion angles.
The inventive method is based on the fact that the sum of the squares of the first noise component "sgr"T and of the second noise component "sgr"P yields the square of the total noise "sgr", "sgr"2="sgr"T2+"sgr"P2.
In a preferred embodiment of the inventive method, the step of resolving the noise part of each pixel into the first noise component and the second noise component is performed by calculating the square of the noise part of each and every pixel for at least two different excursion angles, the values for the square of the noise part acquired in this way defining a straight line in a graph with the square of the noise part on the abscissa and the square of the signal intensity corresponding to the excursion angle on the ordinate, this straight line intersecting the abscissa given an excitation angle of zero degrees, and thus given a signal intensity of zero, and determining the value for the square of the noise part at the intersection of the straight line with the abscissa in order to obtain the value for the square of the first noise component independent of the excitation angle, and determining the slope of the straight line defined in this way and multiplying the slope by the square of the respective signal intensity to obtain the value for the square of the second noise component.
The inventive method is based on the fact that the second noise component "sgr"P is characterized by the signal intensity, which can be modulated by the excitation angle,
"sgr"P=xcexxc2x7s, 
wherein xcex is a constant slope and s is the square of the signal intensity.
The determination of the value for the square of the noise part in the intersection of the straight line with the abscissa in order to obtain the value for the square of the first noise component independent of the excitation angle thereby need not be implemented graphically way but preferably ensues computationally.
The slope of the straight line defined in this way that is determined for each pixel can be employed as a physical criterion for the reduction of the signal-to-noise ratio that respective pixel in the magnetic resonance images with T2* weighting, since a further factor that characterizes the respective pixels is thus acquired.
The step of resolving the second noise component of the noise part of each pixel into the third noise component independent of the echo time and the fourth noise component dependent on the echo time particularly ensues by observing at least two images produced using different echo times.
The third and fourth noise components can be determined especially easily by comparing the two images produced with different echo times.
The inventive method is based on the fact that the sum of the squares of the third noise component "sgr"NB and of the fourth noise component "sgr"B is equal to the square of the second noise component "sgr"P, "sgr"P2="sgr"NB2+"sgr"B2.
In a preferred embodiment, the curve of the third noise component "sgr"NB can be described as "sgr"NBxcx9cS0xc2x7exp(xe2x88x92TExc2x7R2*), wherein R2* is a transversal-relaxation rate contained in an acquired magnetic resonance signal, TE is the echo time of the respective, produced magnetic resonance images and S0 is a start value for an echo time equal to zero of a magnetic resonance signal weighted with an effective relaxation time T2*.
Correspondingly, the curve of the fourth noise component "sgr"B according to this preferred embodiment can be described as "sgr"Bxcx9cS0xc2x7TExc2x7R2*xc2x7exp(xe2x88x92TExc2x7R2*), wherein R2* is a transversal-relaxation rate contained in an acquired magnetic resonance signal, TE is the echo time of the respective, produced magnetic resonance images and S0 is a start value for an echo time equal to zero of a magnetic resonance signal weighted with an effective relaxation time T2*.
The step of resolving the second noise component of the noise part of each and every pixel into the third noise component and the fourth noise component then preferably proceeds by calculating the square of the noise part of each pixel for at least two different excitation angles for at least two different echo times, the values for the square of the noise part acquired in this way for each echo time respectively defining a straight line in a graph with the square of the noise part on the abscissa and the square of the signal intensity proportional to the excitation angle on the ordinate, determining the slope of the straight line defined in this way in order to obtain the square of the third noise component and in order to obtain the square of the fourth noise component, and deriving the third noise component "sgr"NB and the fourth noise component "sgr"B according to the equations "sgr"NBxcx9cS0xc2x7exp((xe2x88x92TExc2x7R2*) and "sgr"Bxcx9cS0xc2x7TExc2x7R2*xc2x7exp(xe2x88x92TExc2x7R2*), wherein R2* is a transveral-relaxation rate contained in an acquired magnetic resonance signal, TE is the echo time of the respective magnetic resonance images and S0 is a start value for an echo time equal to zero of a magnetic resonance signal weighted with an effective relaxation time T2*.
Again, the derivation of the third noise component and fourth noise component by adaptation of the third noise component and fourth noise component to the squares of the slope obtained for each echo time thereby need not ensue graphically but preferably is implemented computationally.
Neural activity changes in the brain of the subject are especially accessible to a further evaluation by a specialist when the step of employing the fourth noise component of the noise part of each pixel for detecting neural activity changes in the brain of the subject includes the step of a spatially resolved presentation of the fourth noise component.
The aforementioned object is also achieved by a magnetic resonance tomography apparatus having a device for spatially resolved presentation of functional activity changes of a brain of a living subject under observation by means of magnetic resonance according to the above-described method, having a control unit that controls the magnetic resonance tomography apparatus for producing temporally successive magnetic resonance images of the brain of the subject stimulated with a stimulus with variation of at least one of the excitation angle and of the echo time; as well as a processing device which calculates a noise part for each pixel referenced to identical pixels of the temporally successive images, in order to resolve the noise part of each pixel into a first noise component independent of the excursion angle and into a second noise component dependent on the excitation angle, and to resolve the second noise component of the noise part of each pixel into a third noise component independent of the echo time and a fourth noise component dependent on the echo time, and to detect neural activity changes in the brain of the subject using the fourth noise component of the noise part of each pixel obtained in this way.
The inventive method thus can be implemented by means of the inventive magnetic resonance tomography apparatus.
In order to facilitate an evaluation of the neural activity changes in the brain of the subject detected by means of the magnetic resonance tomography apparatus by a specialist in an especially appealing way, it is advantageous for the magnetic resonance tomography apparatus also to have a display device that visualizes the neural activity changes in the brain of the subject by means of spatially resolved presentation of the fourth noise component.